Gases that occur in nature or which are produced in industrial processes often contain carbon dioxide in small amounts. For example atmospheric air generally contains about 250 parts per million (ppm) carbon dioxide. Because of certain process constraints or a particular end use that the gas is intended for, it may sometimes be desirable or necessary to remove the carbon dioxide from the gas. For example, air that is separated into various component products by cryogenic separation techniques (cryogenic air separation), such as cryogenic distillation or cryogenic adsorption, must be substantially free of both carbon dioxide and moisture, because these operations are carried out at temperatures below the freezing point of these compounds; consequently, if they are not removed they will freeze in and eventually clog the air separation process equipment.
Small amounts of carbon dioxide and moisture are removed from gas streams by various techniques, such as condensation, reversing heat exchange freezing and adsorption. A particularly preferred method is adsorption using an adsorbent which adsorbs carbon dioxide (and water vapor) more strongly than it adsorbs other components of the gas stream. For example, it is common to remove carbon dioxide from an air stream that is to be cryogenically separated, by passing the gas stream through a bed of zeolite 13X. U.S. Pat. No. 3,885,927, issued to Sherman et al. on May 27, 1975, discloses the use of type X zeolite containing at least 90 equivalent percent barium cations for the removal of carbon dioxide from gas streams containing not more than 1000 ppm carbon dioxide, at temperatures of -40.degree. to 120.degree. F. U.S. Pat. No. 4,775,396, issued to Rastelli et al. on Oct. 4, 1988, discloses the adsorption of carbon dioxide from gas streams by pressure swing adsorption at temperatures of -50.degree. to 100.degree. C., the adsorbent having a SiO.sub. 2 /Al.sub.2 O.sub.3 molar ratio of from 2 to 100 and containing at least 20 equivalent percent of one or more cations selected from zinc, rare earth, hydrogen and ammonium cations and not more than 80 equivalent percent of alkali metal or alkaline earth metal cations.
Zeolite 13X efficiently removes small amounts of carbon dioxide (and water vapor) from air streams at low temperatures, i.e. temperatures of about 5.degree. C. or lower, because it more strongly adsorbs these components than it adsorbs nitrogen, oxygen or argon. However, the carbon dioxide adsorption capacity of zeolite 13X diminishes rapidly as the temperature of the gas being separated increases, and the separation process becomes infeasible at temperatures above about 20.degree. C. Since ambient temperatures are often considerably above the preferred 5.degree. C. adsorption temperature, for example ambient temperatures of 40.degree. C. or higher are sometimes encountered, and since, because of the heat of adsorption and the heat of gas compression, there is a tendency for adsorption bed temperatures to increase considerably during the course of an adsorption process, it is usually necessary to cool air fed to an adsorption-based air prepurification plant by means of external refrigeration to maintain the gas at temperatures below 20.degree. C. This reduces the overall efficiency of the air separation process, since energy must be consumed to provide the necessary refrigeration.
It would be very advantageous to completely eliminate the need for refrigeration or to significantly reduce the amount of refrigeration required in commercial air separation adsorption-based prepurification procedures, since that would enhance the overall economic attractiveness of the air separation process. The present invention provides a novel carbon dioxide adsorption process which provides such an advantage.